Today the BASE collaboration reports on the first measurement of cyclotron quantum heating rates in a cryogenic Penning trap. The team demonstrates that the scaled electric field noise in the spin-analysis trap, an essential instrument in the 1.5 p.p.b. measurement of the antiproton magnetic moment, is much lower than observed in other ion trap experiments. It corresponds to a heating rate below 0.1 quanta per hour and a radial energy stability on the peV/s-level.
Cyclotron transition rates were measured by employing the continuous Stern-Gerlach effect, which couples the radial quantum states to the axial motion of a trapped antiproton. By evaluating the axial frequency stability and comparing it to noise driven random walks in the cyclotron motion, in total 6(1) cyclotron quantum transitions per hour were observed which results in a heating rate below 0.1 quanta per hour. For the electric field noise in the trap, an absolute noise spectral density at the 10^(-20) V^2 m^(-2) Hz^(-1)-level and a scaled noise density below 10^(-11) V^2 m^(-2) is obtained. Compared to Paul-trap and room temperature Penning-trap experiments, the scaled field noise in the cryogenic Penning trap setup is by more than two orders of magnitude smaller.
To understand the origin of these electric field fluctuations, heating rate measurements were conducted at various particle orbits, corresponding to different positions in the trapping potential. Based on these measurements a residual trap potential fluctuations were identified as the dominant source of electric field noise in the experiment. Effects of anomalous heating imposed by fluctuating patch potentials were not resolved within the measurement precision.